Gene targeting by homologous recombination can be used to activate or inactivate cellular genes in eukaryotic cells. The objective of this project is to alter the functional status of genes that control growth and maturation of specific tissues and study the biological consequences of these molecularly defined alterations. The interleukin 3 gene has been chosen for this study as it is a well characterized gene whose biological functions are well known. Activation of the interleukin 3 (IL-3) gene will be attempted in a cell line (32-D) which is IL-3 dependent for growth. Successful activation will therefore lead to a selective phenotype which will minimize screening and identification of the cells where the gene has ben targeted to the correct location. Similarly, the biological importance of the IL-3 gene will be studied in the context of a whole organism by targeting a defective IL-3 gene to its cognate counterpart in embryonic stem cells by homologous recombination. These cells will subsequently be used to generate transgenic mice containing a defective IL-3 gene.